The present invention is related to the field of edge detection and in particular to the field of optical detection of edges and the detection of edges based on the detection of motion.
Detection of the edge of a moving surface is useful in many fields such as printing, in which the position of the edge of a moving sheet of paper at some position or time is often measured to allow for alignment between the sheet and a printing engine.
Edge detectors for sheets in the prior art are based on mechanical or optical detection of the presence of the sheet. For example, in some such detectors, a lever or other indicator may be moved from one position to another by the presence of a sheet. In others an optical sensor determines the presence of a sheet by detecting either the blocking of radiation by the sheet or, alternatively by detecting radiation reflected by the sheet.
An aspect of some preferred embodiments of the invention is concerned with the detection of an edge from detection of its movement.
In preferred embodiments of the inventions, a point or area at which the presence of an edge of a moving surface is to be detected is monitored for movement. Prior to the arrival of the edge, no movement is detected. As the edge passes the point or area, the movement of the edge is detected and the presence thereof is determined. It should be noted that in some preferred embodiments of the invention, the presence of the surface itself is not directly determined. Rather, it is only determined indirectly from a preferably sudden change in the measured state from no measured velocity to a velocity above a certain threshold or within a predetermined range.
In a preferred embodiment of the invention, the velocity is measured optically. In this preferred embodiment of the invention, an optical velocitometer or the like is used to measure velocity (or to determine its presence) at a point, an edge or an area. In principle, any optical velocitometer may be used; however, in preferred embodiments of the invention, specially constructed optical measurement devices are utilized which take account of simplifications that are possible since an accurate measurement of the velocity is not necessary, only of its existence or change relative to some reference. In some preferred embodiments of the invention, to increase accuracy of timing of the passage of the edge, the beam is shaped. This shaping may be to reduce the size of the interrogated area or, more preferably, may provide an edge of radiation at the arrival edge of the surface.
In preferred embodiments of the invention, no measurement of the presence of the presence of the surface is made, unless it is in motion.
In a preferred embodiment of the invention, the edge is the edge of a sheet, such as of paper or plastic. Preferably, the sheet travels along a platen. The detector detects a transition from no motion to motion and the moment of time of transit of the edge is determined from the time of the transition.
In preferred embodiments of the invention, the motion is detected from a Doppler shift of radiation, preferably from a laser, that is reflected from the surface of the moving object (sheet).
There is thus provided, in accordance with a preferred embodiment of the invention a method of detecting the passage of an edge of a moving sheet of material at a position, comprising:
monitoring a state of motion at a position; and
determining a time of an edge of the sheet at a position as a time at which a change in a state of motion at the position is detected.
Preferably, the method comprises:
determining a time of passage of a leading edge as a time at which motion is first detected.
Preferably, the method comprises:
determining a time of passage of a trailing edge of the sheet as a time at which detection of motion ceases.
In a preferred embodiment of the invention, wherein monitoring a state of motion comprises optically monitoring whether motion of an object exists at the position.
Preferably, monitoring comprises determining if a Doppler shift exists between radiation illuminating the position and radiation reflected at the position.
In a preferred embodiment of the invention, the presence of motion is determined to exist if the power of the reflected illumination within a band of Doppler frequencies is above a threshold.
Preferably, the threshold for determining onset of motion is different from the threshold for determining cessation of motion.
Preferably, the method includes providing a local oscillator for determining the Doppler shifts.
In a preferred embodiment of the invention, a portion of the illuminating radiation is used as a local oscillator in detecting Doppler shifts. Preferably, the local oscillator comprises optical radiation reflected from a surface. Alternatively, the local oscillator comprises radiation back diffracted from a diffracting surface.
In a preferred embodiment of the invention, the local oscillator is focused on a sensor used to detect the Doppler shifts. Preferably, the local oscillator is focused essentially to a point.
In a preferred embodiment of the invention, the position is comprised in an area illuminated by radiation, said area comprising an entrance side at which a sheet enters the area and an exit side at which the sheet leaves the area. Preferably, the radiation illuminating the area is formed to provide a sharp illumination boundary at at least one of the entrance and exit sides of said area. More preferably, the radiation illuminating the area is formed to provide a sharp illumination boundary at both the entrance and exit sides of the area.
In a preferred embodiment of the invention, the arrival of an edge is determined by Doppler energy produced at said entrance side of said area by a leading edge of said sheet. In a preferred embodiment of the invention, a trailing edge is determined by a cessation of detection of Doppler energy from said exit side of said position from a trailing edge of said sheet.
In a preferred embodiment of the invention, the position has an extent between entrance and said exit side. Preferably, the extent is between about 1 mm and about 5 mm, more preferably between about 2 mm and about 4 mm and typically, about 2 mm.
In a preferred embodiment of the invention, the method includes detecting the presence of a non-moving sheet between the entrance and exit.
In a preferred embodiment of the invention, the method includes measuring the velocity of the sheet.
In a preferred embodiment of the invention, the method includes measuring the distance the sheet translates.
In a preferred embodiment of the invention, the radiation is IR radiation.
In a preferred embodiment of the invention, the radiation is laser illumination.
In a preferred embodiment of the invention, the time at which a sheet enters the position is determined to an accuracy, better than about 0.5 mm/v sec, and more preferably 0.1 mm/v sec where v is the velocity of the sheet.